Despite advances in the management of the patient with traumatic injury, a large number of such patients die of severe hypovolemia and circulatory collapse due to blood loss. The high mortality may be reduced by a better understanding of mediators responsible for the transition from reversible hypovolemia to circulatory shock so that modulation of these factors can prevent progressive cell and organ damage. Our results indicate that the vascular responsiveness to a recently-reported potent vasodilatory peptide, adrenomedullin (AM), is depressed following hemorrhage. The reduction of a novel AM binding protein (AMBP-1) appears to contribute to the vascular AM hyporesponsiveness. In addition, administration of AM/AMBP-1 in combination with fluid resuscitation improves cardiovascular responses and attenuates organ injury after hemorrhage. We therefore hypothesize that vascular AM hyporesponsiveness due to decreased AMBP-1 likely plays a major role in producing hypovolemic circulatory collapse after hemorrhage. We also hypothesize that administration of AM/AMBP-1 will prevent the transition from the reversible to irreversible hypovolemic shock and will reduce hypovolemia-indueed organ injury and mortality. A rat model of hemorrhage-induced hypovolemia will be used in our 3 specific aims. Aim I: To elucidate mechanisms responsible for producing vascular AM hyporesponsiveness after hypovolemia. We will examine the effects of hemorrhage on AMBP-1 biosynthesis, its binding capacity, AM activity, and AM receptors. The role of proinflammatory cytokines will be determined. Studies are also proposed to examine whether Kupffer cell activation suppresses hepatic AMBP-1 biosynthesis. Aim II: To determine whether delayed administration of AM/AMBP-1 and/or in combination with a reduced volume of resuscitation fluid has any beneficial effects on cell and organ function and survival. Aim III: To investigate mechanisms responsible for beneficial effects of AM/AMBP-1 in hemorrhage-induced hypovolemia. We will examine whether AM/AMBP-1 affects AM receptor and its binding capacity, signal transduction pathways, vascular endothelial cell function, apoptosis, and shedding. The role of proinflammatory and anti-inflammatory cytokines in producing beneficial effects of AM/AMBP-1 will also be tested. The proposed studies will provide useful and novel information which will allow us not only to better understand the mechanisms responsible for the transition from reversible hypovolemia to circulatory collapse after severe blood loss, but also to improve cell and organ function and prevent hypovolemia-induced mortality.